Failed Air Care

My 1998 Windstar recently failed Air Care with a Nitrous Oxide reading in excess of the allowable limit. It also failed the same test 12 months ago and my Ford Service department could not identify or fix the problem at that time. Any suggestions/help would be greatly appreciated.

High NOx is almost always the result of a faulty EGR system. Pull/inspect & clean the EGR valve & make sure the EGR ports & tubing are clear of carbon build-up.
G/luck
Joel